# Condensed Matter Seminar Series

## Emerging Nonadiabatic Wavefunction in Strongly Coupled Qubits

### Soumya Bera

#### Institut Neel, Grenoble France

##### Thursday Feb 27, 11:30 am, Room 298, Physics Building

Abstract: In
nature real systems are coupled to a large number of macroscopic
degrees of freedom which play an important role in determining their
phase coherence. To understand the role of the environment it is
customary to begin with a simple model of a qubit (two level system)
coupled with an infinite number of quantum oscillators (bosons). While
the weak coupling limit of this model is well understood by using
perturbative approaches, a complete analytical theory beyond the
perturbation theory still needs to be addressed. In this work we
present a generalized variational coherent state ansatz for the ground
state of the qubit-photon system, which is supported by constructing
quantum tomography of the states using Numerical Renormalization Group
calculations. We show that at strong coupling the ground state
wave-function of the joint spin-boson system is highly entangled with
emerging non-adiabatic features (Schrodinger cat like states of the
environment). The Wigner distributions of the bosonic wave-function
projected in different spin sectors support this strongly non-adiabatic
nature of the wave-function. Furthermore, we calculate the entanglement
entropy of the spin and a single bosonic mode subsystem. The joint
entropy shows a peak structure around the Kondo scale, which further
confirms the non-polaronic effect in the ground state
wave-function.

*Host: Harold Baranger*